Magneto-optical elements

ABSTRACT

The present invention relates to a novel magneto-optical device composed of a rare-earth chalcogenide and to a method for making such device. When such rare-earth chalcogenides are made in the form of crystals or of thin layers, they are subject to deterioration by exposure to the atmosphere. The use of a thin overlying protective layer having optical and magnetic properties compatible with the underlying rare-earth chalcogenide is highly desirable. In one example, Eu2O3 is employed as the protective layer for EuO.

United States Patent Alm et a1.

[54] MAGNETO-OPTHCAL ELEMENTS [72] Inventors: Kie Y. Ahn, RFD No. 1,Bedford, NY. 10506; Siegfried Methfessel, College Hill, Montrose, NY.10548 [22] Filed: Apr. 28, 1970 [21] Appl. No.: 48,581

Related US. Application Data [621 Division of Ser. No. 603,933, Dec. 22,1966.

[52] US. (31. ..ll7/237, 117/106 R, 117/239, 117/240 [51] Int. Cl. .HOlf110/02 [58] Field ofSearch ..1 17/239, 237, 238, 240, 106R [5 6]References Cited UNITED STATES PATENTS 3,376,157 Guerci et a1. ..340/15lX [151 emmw 1 Eeb. 15, 1972 3,434,863 3/1969 Hansen et a1. ..1 17/106 XPrimary Examiner-William D. Martin Assistant Examiner-Bemard D. PianaltoAttorney-Hanifin and Jancin and George Baron [5 7] ABSTRACT The presentinvention relates to a novel magneto-optical device composed of arare-earth chalcogenide and to a method for making such device.

When such rare-earth chalcogenides are made in the form of crystals orof thin layers, they are subject to deterioration by exposure to theatmosphere. The use of a thin overlying protective layer having opticaland magnetic properties compatible with the underlying rare-earthchalcogenide is highly desirable. In one example, Eu 0 is employed asthe protective layer for EuO.

3 Claims, 5 Drawing Figures 10 VAC. PUMP MAGNETO-OPTICAL ELEMENTS Thisapplication is a division of copending application Ser. No. 603,933,filed Dec. 22, 1966.

CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION Mostrare-earth compounds are highly chemically active and when exposed tonormal atmospheric conditions will deteriorate. Unless a protectivelayer is employed with the rare-earth chalcogenide that is employed aspart of an optical element'or system, the deterioration of suchchalcogenide will diminish the life of the optical element.

However, when a protective layer is used, the latter must have a hightransparency for the light used in conjunction with the rareearthchalcogenide and also must have an index of refraction that matches theindex of refraction of the rareearth chalcogenide being protected. Ifdesired, the thickness of the protective layer can be selected to servealone, or in combination with other transparent layers, to increase themagneto-optical rotation employing the principle of light interference.See Optical Properties of Various Evaporated Rare Earth Oxides andFlourides -G. Haas et al.-JOPS of America, Vol. 49, No. 2, Feb. 1959,pp. 116-120.

SUMMARY OF THE INVENTION The protective layer relied upon in thisinvention is Eu O and in particular rare-earth chalcogenide to beprotected is Eu0, although other rare-earth chalcogenides can be soprotected. Eu has an exceptionally high magneto-optical activity for redlight, e.g., Faraday rotation constant of 6X10 degree/cm. at awavelength of 7,000 A., and it is highly desirable to obtain a thinprotective layer of Eu0 that would be compatible with such rare-earthchalcogenide. Eu O has been discovered to be a protective layer that isparticularly well suited for use with Eu0.

One method for making an Eu0 film having a compatible superimposedprotective layer of Eu O comprises evaporating the Eu O by electronbombardment ofa source of Eu O immediately after the Eu0 layer has beenlaid down. The electron bombardment vaporizes Eu O from the source, thelatter being in the same evacuated chamber as the deposited Eu0 film,and Eu,,0 condenses onto the Eu0 film. The thickness of the Eu Q, filmcan be varied to suit the wavelength characteristics of the light beamused for recording on or reading out information from the Eu0 film.

A second method for producing the Eu O protective film comprisesdepositing a predetermined thickness of Eu0 onto a substrate. During thelast phase of such Eu0 film evaporation, oxygen is bled into theevacuated chamber housing the Eu0 film to cause Eu0 and O to react andproduce a layer of au o,

Thus, it is an object of this invention to obtain a suitable protectivefilm for EuO.

It is yet another object to obtain a protective film for EuO that iscompatible with both the light transmission properties and index ofrefraction of EuO.

A further object is to provide a reliable method for coating a thin filmof Eu0 with a compatible protective layer.

Still another object is to obtain a protective film for a rareearthchalcogenide that is highly transparent and has such a thickness that itincreases rather than perturbs the magnetooptical application of Eu0 atcertain wavelengths of light.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically shows anapparatus for making a protective film of Eu 0 for a film of Eu0 or anyother rare-earth chalcogenide.

FIG. 2 is a slight modification of the apparatus of FIG. 1 forillustrating a second method for obtaining a protective film of Eu 0 onEu0.

FIGS. 3 to 5 are graphs illustrating the effectiveness of Eu 0 as aprotective layer for a magneto-optically active element such as Eu0.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, bell jar 2represents schematically an evacuated chamber housing and the vacuum inchamber 4 is maintained at 1X10 mm. of Hg or better. A substrate 6 ofglass, quartz, or other nonreacting material is conventionally supportedin the chamber 4. The substrate 6 is cleaned prior to being placed inchamber 4 and can be maintained at any temperature between 20 C. to 500C. Eu0 is contained in a refractory crucible 8 and by induction heatingor other means, not shown, the Eu0 is raised to a temperature near orbeyond its melting point, causing Eu0 vapor 10 to condense as layer 12onto substrate 6. Thickness of the Eu0 film 12 can be monitored bywell-known means that are not a part of this invention. When apredetermined or desired thickness of Eu0 has been deposited, theheating means applied to the crucible 8 is cut off or a shutter 14 isinterposed between the Eu0 source and substrate 6.

Soon after the termination of Eu0 deposition, an electron gun source 16is turned on and a slug 18 of Eu 0 is bombarded with electrons. Bybombarding the Eu 0 slug with an input power of approximately 1,000watts or more, a layer 20 of Eu 0 is built upon Eu0 layer 12 at a rateof 5 to 20 angstroms per second.

The finished device comprising substrate 6, Eu0 layer 12 and Eu 0 layer20 can be used in an optical device or system where it is desired toemploy either the Faraday or the Kerr effect. The Faraday effect is thatcharacteristic of a substance which rotates the plane of polarized lightpassing through it when magnetized by an applied magnetic field,provided there is a component of the magnetic field that is parallel tothe polarized light, while the Kerr effect is that characteristic of asubstance which rotates the plane of polarized light reflected upon themagnetized magneto-optical surface. When the Eu 0 protected Eu0 layer ofthis invention is employed to make use of the above mentionedmagneto-optical effect, it is necessary for the substrate 6 to betransparent.

Where it is desired to direct a polarized light beam through themagneto-optically active Eu0 layer, so as to rotate the plane ofpolarization of the light traversing the EuO, then reflect suchtransmitted light back through the Eu0 so as to again rotate the planeof polarization of the light beam, then the substrate 6 need not betransparent but could be made of metal or any other substance that ishighly reflective of the polarized beam making the multiple traversesthrough the Eu0 layer.

FIG. 2 depicts an apparatus that carries out another method offabricating an Eu 0 protective layer 20 onto a magneto-optically activelayer of Eu0 12. The vacuum of chamber 4 is maintained at 1X10 mm. of Hgor better, and the substrate 6 is kept at a temperature of 20 C. to 200C. Induction heating of Eu0 sources 8 is controlled so that Eu0 isevaporated from crucible 8 at a rate of about 10 angstroms/sec. After adesired thickness of Bull has been deposited, oxygen is bled into thechamber 4 through valve 22 at a partial pressure of the order of 10 mm.of Hg. In ten seconds, a total thickness of about angstroms of Eu 0 isdeposited. It is readily seen that one may obtain varying thicknesses ofEu or Eu 0 by changing either the times of deposition or the rates ofdeposition.

A further method of depositing a protective layer of Eu 0 on Eu0 is tocomplete the deposition of EuO layer 12 while maintaining the substrate6 at a temperature of 20 C.200 C. and the vacuum conditions the same asabove. After such completion of EuO deposition, the temperature ofsubstrate 6 is raised to over 200 C. and oxygen is bled into the chamberat a partial pressure of mm. of Hg to react with the deposited Eu0,forming Eu 0 After about ten minutes, a 100 angstrom thick layer of Eu 0is built up on the Eu0 layer 12.

The improved stability of an EuO layer protected by Eu 0 is demonstratedby crystal-structure observations with X-rays.

FIG. 3 is a plot of X-ray intensity of an X-ray beam reflected fromcertain planes of an Eu0 layer protected by Eu 0 versus the Bragg angleat which such reflecting X-ray beam was incident on such planes. Theplot was taken when the relative humidity of the air was about 40percent, and the Eu O protected layer of EuO was exposed for one-halfhour. Curves 30 and 32 show strong presence of EuO, curves 34 and 36show the presence of Eu and curve 38 the presence of Eu 0 FIG. 4 is thesame plot of FIG. 3 but after a time lapse of 17 hours exposure tosubstantially the same atmosphere as the sample of FIG. 1, and it isseen that there is no deterioration of the Eu0 layer and its protectivecoating.

FIG. 5 is the same plot as FIGS. 3 and 4 in air with about 40 percentrelative humidity but after a passage of 55 hours. Peaks P,P show thepresence of Eu(OH) the latter being milky in appearance and exhibitingreduced light transmission. However, since Eu0 protected by Eu 0 isnormally employed at relatively low temperatures, below 50 K., to obtainthe benefits of its high magneto-optical rotations, there is a very lowprobability of Eu 0 becoming Eu(OH) The numbers appearing in parenthesesafter the metal or compound, such such as Eu(200), Eu0(lll), etc.represent the planes from which intensity of X-ray reflection wasobserved.

Eu 0 is particularly suitable for protecting EuO because it is verytransparent in the wavelength range of 2 microns to 0.3 micron.Moreover, it has an index of refraction equal to that of But) so that itis an excellent impedance match for EuO. The protective layer of Eu 0 isexceedingly stable. If desired, one

may build up a plurality of alternate layers of Eu0 and Eu 0 if onerequires many such thin film layers for operation. The thickness of thelayers will be chosen in such a way that a maximum magneto-opticaleffect is obtained by considering light interference effects takingplace in these layers,

Moreover, Eu 0 has very good mechanical characteristics, that is, it isvery hard and not readily scratched or scuffed. It is also understoodthat the protective layer of Eu 0 can be applied to solidmagneto-optically active surfaces. such as crystals, instead ofevaporated thin films.

While the invention has been particularly shown and described withreference to preferred embodiments thereof. it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

l. A method of forming a magneto-optically active element in anevacuated chamber comprising the steps of:

vapor depositing a film of EuO ofa predetermined thickness onto asubstrate, maintaining said substrate at a temperature of 20 C.200

C. during said vapor deposition of EuO, and,

after such deposition, vapor depositing a predetermined thickness of Eu0 onto said film of EuO by bleeding oxygen into said evacuated chamberto oxidize the Eu0 vapor before deposition onto the Eu0 film.

2. A method of forming a magneto-optically active element in anevacuated chamber comprising the steps of:

vapor depositing a thin film of EuO onto a substrate while maintainingthe latter at a temperature of 20 C. to 200 C., raising said substratetemperature to above 200 C. after said depositionhas been completed, andbleeding oxygen into said evacuated chamber to cause the top surface ofsaid Eu0 film to react with said oxygen so as to provide a protectivelayer of Eu O with the initially deposited EuO film.

3. The method of forming a magneto-optically active element as set forthin claim 2 wherein a vacuum of the order of 10 mm. of Hg is used.

2. A method of forming a magneto-optically active element in anevacuated chamber comprising the steps of: vapor depositing a thin filmof Eu0 onto a substrate while maintaining the latter at a temperature of20* C. to 200* C., raising said substrate temperature to above 200* C.after said deposition has been completed, and bleeding oxygen into saidevacuated chamber to cause the top surface of said Eu0 film to reactwith said oxygen so as to provide a protective layer of Eu203 with theinitially deposited Eu0 film.
 3. The method of forming amagneto-optically active element as set forth in claim 2 wherein avacuum of the order of 10 5 mm. of Hg is used.